1. Field of Invention
This invention relates to document forgery protection systems and methods.
2. Description of Related Art
Various techniques are known for detecting and/or deterring forgery of an original printed document. Document forgery includes both unauthorized alteration of the original document and unauthorized copying of the original document. Previously, watermarks have been applied to documents to detect and/or deter forgery. Watermarks are printed marks on a document that can be visually detected or detected using special equipment. Fragile watermarks are marks that appear in an original printed document but that will not appear in a copy of the original document made on a standard photocopier or will be detectably degraded in the resulting copy of the document.
Robust watermarks are marks in an original document that will be accurately reproduced on any copy of the original document made on an standard photocopier so that information contained in the watermark can be extracted from the copy. There are two types of robust watermarks that can be used. The first type of robust watermark is a mark that appears on both the original document and a copy. The second type of robust watermark is a mark that is present, but that is not readily visible, on the original document, but that becomes clearly visible on a copy of the original document. The second type of robust watermark is also known as an invisible robust watermark.
Forgery of an original document containing a fragile watermark by copying the original document is easily detected by the absence of the watermark on the copy of the original document. Forgery of an original document containing the first type of robust watermark is detected by extracting information contained in the robust mark. This information could identify a custodian of the original document and information relating to copy restrictions or other restrictions as to the use of the information in the original document. Forgery of an original document containing the second type of robust watermark is detected by the visible presence of the watermark on the copy of the original document. For example, the information contained in the second type of robust watermark could be a banner that reads “This is a copy” or a similar warning.